Carbon fiber reinforced carbon

from Wikipedia, the free encyclopedia

Carbon fiber reinforced carbon ( English carbon-fiber-reinforced carbon , CFRC, reinforced carbon-carbon , RCC or carbon fiber carbon composite, CFC) are composite materials made entirely of carbon exist.

CFC materials consist of carbon fibers with a diameter of approx. 5–10 µm (≈ 1 ⁄ 15 the diameter of a human hair ), which are embedded in a matrix of pure carbon. They give the material its high mechanical stability. The carbon matrix absorbs external forces and distributes them within the structure .

General

CFC materials can be used at temperatures of up to 3000 ° C under protective gas or in a vacuum . They can withstand extreme temperature shocks, such as immersion in ice water at a temperature of 1500 ° C. CFC materials are also characterized by their very tolerant behavior towards mechanical loads. While traditional ceramics such. If, for example, silicon carbide or pure, unreinforced graphite can quickly break into many parts due to impacts or vibrations due to their brittle behavior, CFC composites can literally be nailed to the wall without bursting. This property of limiting mechanical effects locally, significantly increases the operational reliability of CFC components.

Thanks to its extraordinarily high chemical and thermal resistance, CFC material is suitable for use in which there is contact with highly corrosive, hot media. An example of this are the support grates for rectification columns for hydrofluoric acid recovery . The extremely high purity of CFC materials can also play a major role in some applications. In the semiconductor industry, for example, the crucible supports are made of CFC material in the crystal pulling process.

Manufacturing

CFC materials are manufactured in three steps.

  • First, carbon fibers are fixed in a mold with an organic binder such as plastic or pitch . Carbon-rich aggregates such as coke are often added to the binder in order to accelerate the subsequent charring process.
  • In the second step, the bound material is heated in the absence of air, so that the organic materials pyrolyze to relatively pure carbon . An outgassing process takes place, resulting in a reduction in volume, giving the material a porous structure.
  • In the final step, the pores are closed by vapor deposition of carbon from a gaseous carbon source, such as ethyne , at high temperatures over a period of several days. The heat treatment also causes larger graphite crystals to form in the material. This last step is mainly responsible for the high price of CFC materials, which can potentially exceed $ 100,000 per workpiece.

properties

Like all composite materials, CFC materials combine the typical properties of their material components (matrix material and fiber material ). Their targeted compilation opens up a wide range of options for modifying CFC materials for specific applications. Other points of attack for controlling product properties are special processing technologies and manufacturing methods.

The general property profile of CFC materials can be described as follows:

  • high thermal stability (no embrittlement, no warping)
  • high mechanical strength
  • high thermal shock resistance
  • high mechanical stability (pseudo-ductile fracture behavior)
  • high purity (up to <10 ppm)
  • high chemical resistance
  • low density (approx. 1.6 g / cm³)

Applications

The roots of CFC technology lie in the aerospace industry. CFC is still used today as a material for booster nozzles in rocket engines. The thermal insulation of space gliders such as the space shuttle is also made of CFC materials. However, the range of CFC applications has grown rapidly in recent years. In the meantime, CFC materials have found their way into numerous applications.

For the first time for brakes Carbon plant parts have been developed and used in the brakes of the supersonic aircraft Concorde . When it was ready for practical use in 1971, Dunlop had ten years of research experience thanks to the Concorde, and the brakes could be installed from the fourth production aircraft. Thereafter, CFC brake discs were used in military and civil aircraft.

In the heat treatment of metals , such as hardening, soldering or sintering, frames made of CFC composite materials are increasingly displacing those made of steel , among other things because the latter tend to become brittle in the carbon-containing atmosphere of hardening ovens - the so-called carburization . In addition, steel frames show strong material distortion at the prevailing temperatures of up to 1300 ° C in the hardening process and the constant temperature change. The high specific heat capacity of the racks is another serious disadvantage that drives up energy consumption. Due to their low density (CFC density: approx. 1.6 g / cm³; steel density: 7.9 g / cm³), frames made of CFC material are many times lighter. Due to their very low coefficient of thermal expansion in the direction of the fibers, which is 24 times below the value for steel, frames made of carbon fiber-reinforced carbon do not warp even after several uses in the hardening oven. The thermal distortion of steel frames, on the other hand, is high, so that conventional frames made of steel or high-temperature metals have to be straightened again and again in the hardening shop. CFC frames oxidize above approx. 400 ° C in air, so they are usually only used in reducing furnace atmospheres or in a (technical) vacuum.

In the hollow glass industry , CFC is used as a substitute for asbestos . Wherever glowing glass is moved ( hot end handling ), CFC can be found as a contact material in rollers, guides and grippers.

In the semiconductor industry , the focus is on the extreme purity of CFC materials. The content of foreign atoms reaches values ​​below 10 ppm and is required in some areas of application. In crystal growth, for example, high-purity CFC outer crucibles support those made of quartz glass that absorb the molten silicon . With the help of a seed crystal , meter-long single crystals ( ingots ) are created. In addition, CFC materials also serve as resistance heaters in such furnaces and as a shield for thermal radiation.

A very modern and at the same time extreme application example for CFC material is its use as a first-wall lining of fusion reactors . Inside these reactors, temperatures of 100 million ° C are required to start nuclear fusion . The plasma is extremely sensitive to impurities, so that very few materials are even suitable for this application. The very favorable mix of high temperature resistance, thermal conductivity, mechanical strength and purity ensures that CFC fiber composite ceramics have established themselves.

The most important applications of CFC materials at a glance:

  • Aerospace Engineering
  • Reactor technology
  • Apparatus construction
  • Furnace construction
  • Semiconductor industry
  • Hollow glass industry
  • Heat treatment
  • Sintering
  • Soldering (hard / high temperature soldering)
  • Medical technology

Individual evidence

  1. ^ Carbon Brakes for Concorde , Flight International, December 30, 1971, p. 1031
  2. ↑ Handling robots ensure a competitive edge. Retrieved October 8, 2017 .
  3. ↑ Material science aspects of the development of novel workpiece carriers for high-temperature processes made of fiber composite ceramics C / C and other high-temperature materials. Retrieved October 8, 2017 .

Web links